Arrangement of electrical components to define a low inductance plasma generating apparatus



Sept. 23, 1969 G, w, FLINT ETAL 3,469,144

ARRANGIMMENT OF ELECTRICAL COMPONENTS TO DEFINE A LOW INDUCTANCB PLASMA GENERATING APPARATUS Filed Oct. 4., 1965 I40 ll '8 '7 n |& 22

18 Ilb FIG. 2

FIG. 3

INVENTORS GRAHAM W. FLINT JAMES R. BOWEN United States Patent 3,469,144 ARRANGEMENT OF ELECTRICAL COMPONENTS TO DEFINE A LOW INDUCTANCE PLASMA GENERATING APPARATUS Graham W. Flint, Winter Park, and James R. Bowen,

Orlando, Fla., assignors to Martin-Marietta Corporation, Middle River, Md., a corporation of Maryland Filed Oct. 4, 1965, Ser. No. 492,664 Int. Cl. H05b 37/00 US. Cl. 315182 9 Claims ABSTRACT OF THE DISCLOSURE The present invention involves a low inductance electrical circuit for obtaining a rapidly rising electrical current to generate a pinched plasma within a non-conducting tube containing a quantity of ionizable gas, comprising an inductance coil for encircling the tube, and capacitor means and switch means closely adjacent the coil. The coil is constituted in two portions, and parallel plate feed lines separately connect one end of each coil portion to opposite terminals of the capacitor means. The switch means are disposed at a different location with respect to the coil than the capacitor means, and are connected to the other ends of the coil portions. Significantly, the electrical interconnections between the switch means and the coil, and the capacitor means and the coil are maintained in a short and closely spaced relationship, thus to represent a comparatively small inductance and making possible a correspondingly high rate of increase of flow of current such as is desirable for creating a pinched plasma in the tube.

This invention relates to a low inductance circuit and more particularly to a low inductance electrical circuit for obtaining a rapidly rising electrical current to generate a pinched plasma within a non-conducting tube.

The pinch elfect is commonly used to obtain high temperatures and high-intensity light in a tube or bottle containing a plasma and may be defined as the self-contraction of a plasma carrying a large current due to the interaction of such current with the magnetic field it produces. The pinch eifect has been used in attempts to obtain thermonuclear reactions and more recently in flash tubes for lasers. Flash tubes are of course laser light sources used to raise the energy level of the active material in the laser. Lasers have been produced where the lasering material is inserted inside of a pinched plasma flash tube. The intense light generated as the plasma contacts is used to raise the energy level of the lasering material. It has also been suggested to use the pinched plasma as the lasering material and thus eliminate the need for a separate material.

Devices generally used for generating a pinched plasma consist of a low inductance driving coil which encircles a non-conducting tube containing a gas at low pressure. For intense light generating applications the tube is usually made of quartz and is filled with a heavy gas such as argon. The coil is connected to a low inductance storage capacitor by means of a switch which is discharged to cause a rising current in the coil and thereby in the gas within the tube by induction. In order to efficientlybreak down the gas and achieve high peak power it is necessary that the initial rise in driving coil current should be very rapid. This requires that the total circuit inductance should be low and that a very small fraction of that inductance "ice resides in components other than the driving coil. In the past devices have been used employing a low inductance capacitor, typically 1020 nanohenries, a parallel plate feed line, a driving coil with an inductance of approximately 50 nanohenries and an external spark gap switch, connected between the capacitor and the coil. The separate spark gap switch adds the order of another 50 nanohenries of inductance to the circuit so that the external circuit inductance constitutes more than half the total inductance.

The present invention uses a technique of combining the driving coil or inductance and spark gap switch in a single unit. This novel combination produces the new and unexpected result of reducing the effective inductance of the feed line and switch to approximately 10% of the total inductance. The new circuit comprises a coil encircling a tube filled with an ionizable gas and a switch for interrupting the continuity through the coil. In accordance with this invention, the switch is disposed within the windings of the coil so as to advantageously lessen the inductance in the manner mentioned hereinbefore. In this context the coil has continuity only when the switch is in the closed position. However, it may be said that the switch is normally an open spark gap switch, whose electrodes enable the flow of current when by the action of a triggering means, current is caused to flow momentarily across such electrodes. A storage capacitor supplies the necessary surge of current to produce a plasma within the tube.

The present invention has the additional advantage of having the spark gap switch in close juxtaposition to the bottle of gas and thereby preionizing the gas within the bottle by the ultra-violet radiation given ofl by the spark gap switch. This eliminates the need in prior art devices of pre-ionizing the gas with a separate radio frequency source. It should be noted, however, that switches other than spark gaps may also be used.

Other objects, features, and advantages of this invention will be apparent from a study of the written description and the drawings in which:

FIGURE 1 is a perspective view of the present invention;

FIGURE 2 is a cross cut view of FIGURE 1 taken along lines 2-2;

FIGURE 3 is a simplified circuit diagram of the present invention.

In FIGURES 1 and 2 one example of the present invention is illustrated at 10 with a generally pancake shaped spark gap switch 11 which has ports 12 for purging the switch of air and thereafter pressurizing the switch with an inert gas such as nitrogen, and a trigger 13 for triggering the switch 11. The switch 11 has upper and lower portions 11a and 11b of suitable metal, such as brass, which portions connect directly to the upper and lower portions 14a and 1412, respectively, of the coil 14 as shown in FIGURE 2. Coil 14 encircles tube 15, which tube is filled with an ionizable gas, such as argon. The coil is energized by discharging a capacitor through the parallel plate feed line 16.

The switch 11 may be purged and then filled with an inert gas such as nitrogen through ports 12 which enter chamber 17 where upper and lower electrodes 18 are located. Electrodes 18, are preferably made of tungsten alloy to withstand the repeated sparking of current flowing between the electrodes. The switch is electrically divided preferably by a thin strip of insulation 19 such as Teflon which may also separate the parallel plate feed lines 16.

This dividing insulation of course has an aperture between the spark gap electrodes. The coil 14 may be made of several turns but a single turn coil has generally been found desirable, and the tube 15, which will normally be made of quartz. The storage capacitor 22 is of a low inductance type and may be a single capacitor or a bank of any number of capacitors and may be charged by any desired means.

In operation, the switch 11 is triggered by the trigger 13 which is activated by any desired means such as an automobile transistorized ignition system, so as to cause current to flow across electrodes 18. The current rises in the coil 14 including an electric field of sufiicient intensity to break down the ionizable gas in the vicinity of the wall of tube 15. As the current in the coil 14 increases, the conductivity of the gas along the wall of the tube 15 increases and a current is developed in the gas which has a direction of flow reversed with respect to the current in the coil 14. The magnetic fields associated with the currents in the coil 14 and the plasma interact such that the plasma ring along the wall is driven inwardly toward the axis 21 of the tube 15. The contracting plasma is confined by the magnetic fields so that plasma temperature in excess of those possible with conventional electrode flash tubes may be used. The compression of the ionized gas also results in increased electron temperature, and an increased electron/ion collision frequency. Consequently, an intense light is produced in tube 15 and used as desired.

FIGURE 3 represents a simplified showing of the invention in circuit diagram form. The storage capacitor 22 stores the energy for the system and is connected to the coil 24 by a parallel plate feed line 25. The coil 24 encircles the gas filled bottle 26 and has a switch 27 interrupting the continuity of the coil 24. The switch 27 is located approximately half way around the coil 24 in the present example but may be located at any point on the coil winding without departing from the scope of the invention.

This invention is not to be construed as limited to the particular forms disclosed herein, since these are to be regarded as illustrative rather than restrictive. For instance, switching means other than a spark gap switch may be used, the bottle or tube may be made of material other than quartz, the switch may be of other and ditferent shapes than those illustrated, and any type of feedline may be utilized without departing from the scope of the invention.

We claim:

1. A low inductance electrical circuit used to generate a pinched plasma within a tube comprising in combination an inductance encircling a tube filled with a quantity of ionizable gas, switch means on one side of the tube for interrupting the continuity through said inductance, and capacitor means on the other side of the tube and also connected to said inductance, the interconnection between said switch means and said inductance, and said capacitor means and said inductance being very short and closely spaced, thus to define a minimum inductance arrangement, whereby the triggering of said switch means will discharge the stored electrical energy through said inductance and induce a pinched plasma in said tube.

2. The circuit according to claim 1 in which said inductance is a coil.

3. The circuit according to claim 2 in which said coil has a single turn.

4. The circuit according to claim 3 in which said switch is located approximately half way around said single turn coil.

5. The circuit according to claim 4 in which said switch means utilizes a spark gap between two electrodes triggered by a third electrode.

'6. A low inductance electrical circuit for obtaining a rapidly rising electric current to generate a pinched plasma within a non-conducting tube containing a quantity of ionizable gas, comprising an inductance coil for en- .4 circling the tube, said coil being constituted in two portions, with each coil portion encircling only a portion of the tube, parallel plate feed lines for separately connecting one end of each of said coil portions to opposite terminals of an energy source disposed closely adjacent said coil, switch means disposed close to said coil at a different location than said energy source, and being connected to the other end of each of said coil portions, the electrical interconnections between said switch means and said coil, and said energy source and said coil, being maintained very short and closely spaced, thus to represent a very small inductance and making possible, when said switch means is closed, a large, rapidly rising current bringing about the creation of a pinched plasma in the tube.

'7. A low inductance electrical circuit for obtaining a rapidly rising electric current to generate a pinched plasma within a non-conducting tube containing a quantity of ionizable gas, comprising an inductance coil for encircling the tube, said coil being constituted in two portions, with each coil portion encircling only a portion of the tube, and with the two ends of one coil portion adjacent the two ends of the other coil portion, parallel plate feed lines for separately connecting one pair of adjacent ends of said coil portions to opposite terminals of an energy source disposed closely adjacent said coil, said feed lines being close together and separated only by comparatively thin insulation means, switch means disposed close to said coil at a different location than said energy source, thus to avoid physical interference with said energy source, and being connected to the other pair of adjacent ends of said coil portions, said switch means being arranged, when closed, to complete an electric circuit such that a large, rapidly rising current from said energy source can flow through said coil and switch means; the electrical interconnections between said switch means and said coil, and said energy source and said coil, being maintained very short and closely spaced, thus to represent a very small inductance and make possible correspondingly high rate of increase of flow of current such as is necessary for creating a pinched plasma in the tube.

8. A low inductance electrical circuit for obtaining a rapidly rising electric current to generate a pinched plasma within a non-conducting tube containing a quantity of ionizable gas, comprising an inductance coil for encircling the tube, said coil being constituted in two portions, with each coil portion encircling only a portion of the tube, parallel plate feeding lines for separately connecting the one end of each of said coil portions to opposite terminals of an energy source disposed closely adjacent said coil, switch means disposed close to said coil at a different location than said energy source, and being connected between the other ends of said coil portions, said switch means being constituted by two principal portions substantially symmetrically disposed with respect to the plane of the connection with said other ends of said coil portions, thus to lessen the inductance of said switch, the electrical interconnections between said switch means and said coil, and said energy source and said coil, being maintained very short and closely spaced, thus to represent a very small inductance and making possible, when said switch means is closed, a large, rapidly rising current bringing about the creation of a pinched plasma in the tube.

9. A low inductance electrical circuit for obtaining a rapidly rising electrical current to generate a pinched plasma within a non-conducting tube containing a quantity of ionizable gas, comprising an inductance coil for encircling the tube, said coil being constituted in two portions, with each coil portion encircling only a portion of the tube, and with the two ends of one coil portion adjacent the two ends of the other coil portion, parallel plate feed lines for separately connecting one pair of adjacent ends of said coil portions to opposite terminals of an energy source disposed closely adjacent said coil, said feed lines being close together and separated only by comparatively thin insulation means, switch means disposed close to said coil at a difierent location than said energy source, thus to avoid physical interference with said energy source, and being connected between the other pair of adjacent ends of said coil portions, said switch means being constituted by two principal portion substantially symmertically disposed with respect to the plane of the connection with said other pair of adjacent ends of said coil portions, thus to lesen the inductance of said switch, said switch means being arranged, when closed, to complete an electric circuit such that a large, rapidly rising current from said energy electrical interconnections between said switch means and said coil, and said energy source and said coil, being maintained very short and closely spaced, thus to represent a very small inductance and make possible corre- A source can flow through said coil and switch means; the 15 spondingly high rate of increase of flow of current such as is necessary for creating a pinched plasma in the tube.

References Cited UNITED STATES PATENTS 2,939,048 5/1960 Waniek 313-161 X 2,939,049 5/1960 Blackrnan 31316l X 3,030,547 4/1962 Dike et a1 313197 X 3,055,262 9/1962 Ducati et a1. 313-241 X 3,087,092 4/1963 Lafierty 313197 X 3,089,831 5/1963 Kolb 313161 X 3,290,219 12/1966 Hurw-itz 313161 X JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner US. Cl. X.R. 

